Melanin Photosensitization and the Effect of VisibleLight on Epithelial CellsOrlando Chiarelli-Neto1, Alan Silva Ferreira1, Waleska Kerllen Martins1, Christiane Pavani1,

Divinomar Severino1, Fernanda Faiao-Flores2, Silvya Stuchi Maria-Engler2, Eduardo Aliprandini3,

Glaucia R Martinez3, Paolo Di Mascio1, Marisa H. G. Medeiros1, Maurıcio S. Baptista1*

1 Departamento de Bioquımica, Instituto de Quımica, Universidade de Sao Paulo, Sao Paulo, Brazil, 2 Departamento de Analises Clınicas, Faculdade de Ciencias

Farmaceuticas-USP, Sao Paulo, Brazil, 3 Departamento de Bioquımica e Biologia Molecular, Setor de Ciencias Biologicas, Universidade Federal do Parana, Curitiba, Brazil

Abstract

Protecting human skin from sun exposure is a complex issue that involves unclear aspects of the interaction between lightand tissue. A persistent misconception is that visible light is safe for the skin, although several lines of evidence suggestotherwise. Here, we show that visible light can damage melanocytes through melanin photosensitization and singletoxygen (1O2) generation, thus decreasing cell viability, increasing membrane permeability, and causing both DNA photo-oxidation and necro-apoptotic cell death. UVA (355 nm) and visible (532 nm) light photosensitize 1O2 with similar yields,and pheomelanin is more efficient than eumelanin at generating 1O2 and resisting photobleaching. Although melanin canprotect against the cellular damage induced by UVB, exposure to visible light leads to pre-mutagenic DNA lesions (i.e., Fpg-and Endo III-sensitive modifications); these DNA lesions may be mutagenic and may cause photoaging, as well as otherhealth problems, such as skin cancer.

Citation: Chiarelli-Neto O, Ferreira AS, Martins WK, Pavani C, Severino D, et al. (2014) Melanin Photosensitization and the Effect of Visible Light on EpithelialCells. PLoS ONE 9(11): e113266. doi:10.1371/journal.pone.0113266

Editor: John R. Battista, Louisiana State University and A & M College, United States of America

Received August 1, 2014; Accepted October 6, 2014; Published November 18, 2014

Copyright: � 2014 Chiarelli-Neto et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permitsunrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

Data Availability: The authors confirm that all data underlying the findings are fully available without restriction. All relevant data are within the paper and itsSupporting Information files.

Funding: FAPESP (12/50680-5; 13/07937-8), CNPq, INCT Redoxoma (FAPESP/CNPq/CAPES; Proc. 573530/2008-4) and FARMA Service Bioextract are acknowledgedfor providing funds for this research. OCN acknowledges FAPESP for his PhD fellowship (2010/08796-0). The funders had no role in study design, data collectionand analysis, decision to publish, or preparation of the manuscript.

Competing Interests: The authors have declared that no competing interests exist.

* Email: baptista@iq.usp.br

Introduction

Humans and other animals produce melanin mainly for

protection against exposure to ultraviolet B (UVB) radiation [1].

In contrast to UVB radiation, which is directly absorbed by DNA,

UVA radiation acts essentially through photosensitization, which

generates a triplet species, 1O2, and subsequently generates other

radical species that can damage both DNA and other epithelial

cell biomolecules [2–4]. UVA penetrates deeper in the dermis

than does UVB, and it is the major UV source responsible for skin

photoaging and the development of several types of skin cancer

[5].

However, the visible portion of the spectra has garnered much

less attention, despite several scientific reports that have described

the effect of visible and IR irradiation on the skin [6,7]. Kielbassa

and co-workers [8] and Kvam and Tyrrel [9] showed that

irradiating Chinese hamster cells and dermal fibroblasts, respec-

tively, with UVA and visible light induced oxidative damage in

DNA. More recent studies showed that visible light disturbs the

epidermal barrier, and this disturbance induces pigmentation and

inflammatory responses [10,11]. However, a great deal of

controversy remains concerning the effect of visible light on the

skin, most likely because of the lack of a mechanism that explains

the observed effects [12].

It has been shown that, in addition to UV [13], visible light also

induces pigmentation in certain skin types. Mahmoud and co-

workers [11] showed that visible light induces skin darkening in

people with skin types IV and V but not in individuals with type II

skin. The darkening induced by visible light depended on the pre-

irradiation melanin content of the skin, suggesting that melanin

may directly damage skin cells upon exposure to visible light.

The literature describes both protective and damaging roles for

melanin [14,15]. Two independent studies using Xiphophorus,which is a fish that is highly susceptible to melanoma, showed that

the action spectra for both melanoma induction [16] and the

photo-induced generation of reactive species [17] extend to visible

wavelengths and that the shapes of the action spectra correspond

to the shape of the melanin absorption spectrum. Despite the lack

of a mechanistic explanation for the observations, these articles

highlight the importance of understanding the role of the excited

species that are generated after melanin is excited by visible light.

In an earlier publication, we showed that melanin can act as a

photosensitizer, leading to 1O2 generation after excitation with

visible light [18]. Singlet oxygen can react with proteins, nucleic

acids and membranes [19] (Figure S1); consequently, melanin

photosensitization is likely involved in the phototoxicity of visible

light, which is the main hypothesis that we aim to demonstrate

herein.

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Methods

ReagentsAll solvents were spectroscopic grade. Water was distilled from

an all-glass apparatus and further purified via a Millipore Milli-Q

system. D2O (99%), tyrosine (Tyr), ammonium chloride (NH4Cl),

the enzymes Fpg and Endo III, 3-(4,5-dimethylthiazol-2-yl)-2,5-

diphenyltetrazolium bromide (MTT), KCl, Na2EDTA, HEPES

and BSA from Sigma-Aldrich (either USA or Germany) were used

as received. All other materials were the best analytical grade

available. NaOD was prepared using three cycles of dissolution

and evaporation of the initial NaOH solid (1 g) with D2O (10 g).

The naphthalene derivative endoperoxide (DHPNO2) was syn-

thesized by photosensitizing the precursor N, N9-di(2,3-dihydrox-

ypropyl)-1,4-naphthalenedipropanamide (DHPN) with methylene

blue as described by C. Pierlot et al [20]. The endoperoxide

concentration was determined for each aliquot after the final

filtration. The solutions were obtained at concentrations between

170 to 190 mmol/L with a purity of approximately 84%. This

purity value refers to the amount of DHPNO2 obtained relative to

the total amount of (DHPNO2+DHPN).

Eumelanin and pheomelanin were synthesized as described by

Haywood et al with modifications [21]. Eumelanin was prepared

from L-tyrosine (2.5 mg/mL) in pH 7.4 phosphate buffer (50 mM)

and mushroom tyrosinase (150 U/mL) in bovine serum albumin

(BSA) solution (5 mg/mL). Pheomelanin was synthesized from L-

dopa (0.5 mg/mL) and L-cysteine (1.5 mg/mL) in pH 7.4

phosphate buffer (50 mM) and mushroom tyrosinase (100 U/

mL). The reactions were performed at room temperature with

stirring for 24 h. We also used eumelanin and pheomelanin

samples that were kindly provided by Dr. S. Ito [22]. The samples

from our lab and those from Dr. Ito’s lab behaved identically with

regard to 1O2 generation.

EquipmentThe visible and UV light irradiator (Novatecnica, Brazil)

included temperature and humidity sensors. Both variables were

maintained during the experiments. The irradiation (mW/cm2)

was measured at eight different areas in this irradiator using a

dosimeter (VLX-3W, France). The irradiances of the light source

were 3.0 mW.cm22 and 3.3 mW.cm22 in the UVB and visible

regions, respectively. For UVB, 25 min of irradiation provided

4.5 J.cm22. For the visible region, 30 min of irradiation provided

6 J.cm22, 180 min of irradiation provided 36 J.cm22, and

360 min of irradiation provided 72 J.cm22.

Cell absorption/emission was quantified using a plate reader

(Tecan Infinite 200M USA). The 1O2 measurements were

performed in a specially designed instrument [18,19,23] consisting

of a Surelite III laser (355 nm and 532 nm, 5-ns pulses, 10 pulses/

s, 1 mJ/pulse; Continuum Lasers), cuvette holder, silicon filter,

monochromator, liquid-nitrogen-cooled near infrared photomul-

tiplier tube (NIR-PMT R5509) from Hamamatsu (Hamamatsu

Co., Bridgewater, NJ, USA) and a fast multiscaler analyzer card

with a resolution of 5 ns/channel (MSA-300; Becker & Hickl,

Berlin, Germany). The signal was acquired either from a cell

cuvette or directly under a fluorescence microscope (Nikon Eclipse

Ti, USA). Fluorescence/transmission microscopy images were

acquired from an Axiovert 200 microscope or an LSM 510 laser

confocal microscope (Zeiss, Germany). The comet assay images

were obtained using fluorescence microscopy (Olympus BH-2,

USA). ImageJ Launcher was used for the confocal image analyses

(National Institutes of Health, Bethesda).

Cell cultureSeveral cell lines, which are available commercially, were

received as gift: B16–F10 [24]; HaCaT [25]; J774 [26] and SK-

mel 28 [27]. B16-F10, HaCaT and Hela cells were cultivated in

Dulbecco’s Eagle (DMEM) culture medium (Sigma-Aldrich). J774

and SK-mel cells were cultivated in RPMI 1640 culture medium

(Sigma-Aldrich). Both media were supplemented with 10% SFB

(Gibco/BRL Life Technologies), 4 mM L-glutamine (Sigma

USA), 100 U/mL penicillin (Sigma USA) and 100 mg/mL of

streptomycin (Nissui Seiyaku) and incubated at 5% CO2 and

37uC. Primary skin cell cultures (melanocytes) were obtained from

the foreskins of University Hospital (Hospital Universitario – HU-

USP) patients [28]. The project was reviewed and approved by the

Research Ethics Committee of the University Hospital (Av. Prof.

Lineu Prestes, 2.565-Cidade Universitaria-CEP 05508-000; +(5511)30919200, Sao Paulo, Brazil) (protocol# 943/09). The

experiments were performed with each subject’s understanding

and written consent, and the study methodologies conformed to

the standards set by the Declaration of Helsinki. The melanocytes

were maintained in 254CF medium (SKU# M-500-254CF;

Cascade Biologics, USA) with human melanocyte growth supple-

ment (HMGS – SKU# S-002-5; Cascade Biologics, USA).

Melanogenesis, irradiation and viabilityThe cells were plated (26104 cells.mL21) and, after 24 h, were

treated with 0.5 mM Tyr (Sigma-Aldrich, Germany) and 10 mM

NH4Cl (Labsynth, Brazil) for 48 h. This protocol increases the

melanin production of melanocytes, and the resulting melanocytes

and are referred to as M++++ herein; the control cells are referred

to as CT. The cells were irradiated in PBS (8 g/L NaCl, 0.20 g/L

KCl, 1.15 g/L Na2HPO4, and 0.2 g/L KH2PO4). We applied

4.5 J.cm22 of UVB irradiation and 36 J.cm22 and 72 J.cm22 of

visible irradiation. The cell density was evaluated using acridine

orange fluorescence (excitation 488 nm, emission 515 nm). The

cell viability was evaluated using MTT colorimetric and crystal

violet assays [29]. Damage to the cytoplasmic membrane was

quantified using propidium iodide incorporation. Apoptotic cell

death was characterized using caspase-3 activation (Cell Signaling

Technology, USA).

B16-F10 cell viability after 24 h of a DHPNO2 treatmentThe B16-F10 cells (CT and M++++) were treated with a

solution containing RPMI and 10 mM DHPNO2 in the absence

of serum for 2 h. In control assays, the cells were treated with a

solution containing RPMI and 10 mM DHPN (decomposition

product of DHPNO2) in the absence of serum for the same time

period. After this period, the cell medium was changed to a normal

culture medium (RPMI 1640 (Cultilab) supplemented with fetal

calf serum (7.5%) from Gibco and with sodium bicarbonate

(0.8 mM), HEPES (20 mM), and gentamicin (50 ng/mL) from

Sigma-Aldrich. Cell viability was determined using an MTT assay

24 h after the DHPNO2 treatment.

Melanin quantificationThe melanin content was quantified as previously described

[30]. The B16-F10 cells were seeded in 96-well plates (16104

cells.mL21), and after 24 h, they were treated with 0.5 mM Tyr

(Sigma-Aldrich Germany) and 10 mM NH4Cl (Labsynth Brazil)

for 48 h. After incubation, a portion of the cells was centrifuged

and suspended in 1 M NaOH (Labsynth Brazil). The other

portion was maintained in PBS (8 g/L NaCl, 0.20 g/L KCl,

1.15 g/L Na2HPO4, and 0.2 g/L KH2PO4) for protein quanti-

fication. Both aliquots were lysed in a Branson Sonifier 450 (USA)

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at 20 W for 30 sec. The melanin was quantified by measuring the

absorption at 470 nm in a Tecan Infinite 200M plate reader using

a standard curve for commercial melanin (Sigma Aldrich

Germany) [30]. The total protein content was determined using

the Bradford method [31]. Melanin is expressed as mg of melanin/

mg of protein.

Comet assayComet assays were performed according to optimized protocols

[32,33]. B16-F10 cells were plated at 16105 cells.mL21 in culture

medium. Twenty-four hours after seeding, the cells were treated

with 0.5 mM Tyr (Sigma-Aldrich, Germany) and 10 mM NH4Cl

(Labsynth, Brazil) for 48 h. After incubation, the cells were

irradiated in PBS (8 g/L NaCl, 0.20 g/L KCl, 1.15 g/L

Na2HPO4, and 0.2 g/L KH2PO4) using 36 J.cm22 or 6 J.cm22

of visible light. We mixed 30 ml of a 16105 cell suspension with

100 ml of agarose (Sigma USA) (0.5% concentration in PBS) and

distributed the mixture on slides that were pre-coated with agarose

(Sigma USA) (1.5% concentration in PBS) and incubated on ice.

After solidifying, the cells were lysed in the dark using a high-salt

alkaline buffer (0.5 M NaCl, 0.1 M EDTA, 0.01 M Tris, and 1%

Triton X-100, pH 10). For samples irradiated with 36 J.cm22 or 6

J.cm22, the slides were placed in electrophoresis buffer (0.3 M

NaOH and 1 mM EDTA, pH 13, cooled in a refrigerator) in the

dark for 30 min. Electrophoresis was performed in a cold-storage

room, in the dark, using a power supply (ESP 301; GE USA) with

the same buffer for 30 min at 25 V. After electrophoresis, the

slides were neutralized using 0.4 M Tris at pH 7.5 and fixed in

ethanol. In the protocol used to evaluate the direct DNA damage

from melanin photosensitization, the cells were treated with

6 J.cm22 of visible light, and before electrophoresis, the slides were

treated with 0.2 U of Fpg or Endo III enzymes (Sigma-Aldrich,

USA) in buffer (0.1 M KCl, 0.5 mM Na2EDTA, 40 mM HEPES

and 0.2 mg/mL BSA at pH 8.0) for 30 minutes at 37uC.

Subsequently, the DNA was stained with ethidium bromide

(10 mg.mL21), excited at 515 nm and observed using a fluores-

cence microscope (Olympus BH-2, USA) [34].

Figure 1. 1O2 generation from melanin. (A) 1O2 decay from a eumelanin solution (0.04 g.L21) in acetonitrile, pD = 10 with excitation at either 355or 532 nm. The 1O2 decay lifetimes were the same under both conditions (,17 ms, which is the lifetime expected for a mixture of acetonitrile andwater) [18]. The insert shows the near infrared emission spectra after exciting a eumelanin solution at 532 nm and 355 nm in the absence of azide(continuous lines) and in the presence of 3 mM azide (dashed flat line); for the excitation pulses at both 355 nm and 532 nm, we used the followingparameters: 5 ns, 10 pulses/s, and 1 mJ/pulse. (B) An integral of the emission spectra from pheomelanin (OD = 1.02) and eumelanin (OD = 0.97)solutions in acetonitrile immediately following dissolution (dark) and after 60 min of irradiation with visible light (light dose of 12 J.cm22). (C)Absorption as a function of irradiation time for pheomelanin (N) and eumelanin (&) solutions in acetonitrile.doi:10.1371/journal.pone.0113266.g001

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1O2 generationThe B16-F10 cells were seeded in six-well plates (26105

cells.mL21), and after 24 h, they were treated with 0.5 mM Tyr

(Sigma-Aldrich, Germany) and 10 mM NH4Cl (Labsynth, Brazil)

for 48 h. The cells were washed in PBS (8 g/L NaCl, 0.20 g/L

KCl, 1.15 g/L Na2HPO4, and 0.2 g/L KH2PO4), removed from

the plates using a cell scraper and resuspended in D2O saline

solution. We obtained 1O2 emission spectra by measuring the

emission intensities from 1180 to 1360 nm with 1- to 5-nm steps

using the equipment described above.

Statistical analysisThe experiments were performed with least as three indepen-

dent repetitions. The statistical analyses were assessed using

Student’s t test and Microcal Origin software (version 7.0); P,0.05

was considered statistically significant.

Results

The excitation of melanin with either UVA (355 nm) or visible

(532 nm) light generated the characteristic 1O2 NIR emission

spectra with a maximum centered at 1270 nm, which is a

fingerprint of the O2 (a1Dg) { O2(X3Sg2) transition (Fig. 1).

Sodium azide suppression confirmed this assignment (Fig. 1B,

insert) [18,19]. The emission intensity was stronger after melanin

was excited by UVA than after excitation by visible light, but this

difference is due to the greater melanin absorption at 355

compared with that at 532 nm; thus, the 1O2 generation efficiency

does not vary depending on source of irradiation (UVA or visible).

Furthermore, the 1O2 emission from pheomelanin was 30%

more intense than that from eumelanin (Fig. 1B), and visible light

irradiation substantially decreased the absorption (Fig. 1C) and the

generation of 1O2 by eumelanin (Fig. 1B); in contrast, the decrease

(,7%) in 1O2 generation by pheomelanin was substantially

smaller (Fig. 1B). The chemical reaction underlying eumelanin

photobleaching is the addition of 1O2 to the double bond at the C3

of the indole group with the consequent hydroperoxide formation

[18]. This type of photoproduct was not detected from the

pheomelanin photolysis.

To understand the potential effects of melanin photosensitiza-

tion on epithelial cells, we compared the UVB and visible light

photosensitivity using cell lines that express different amounts of

melanin (Fig. 2). The irradiation doses were selected to mimic the

exposure of an individual to ,10 minutes of a sunny day in Brazil.

Notably, the cells expressing more melanin had a higher survival

rate after they were challenged with UVB, which is consistent with

previous results [30]. However, the darker cells suffered from high

phototoxicity from visible irradiation (Fig. 2), providing strong

evidence that the phototoxicity from visible irradiation is related to

the amount of melanin.

We then tested the visible light toxicity in two melanocompetent

cell lines (B16-F10 and human Caucasian melanocytes) under two

different melanin production regimens (Fig. 3; i.e., basal level or

control (CT) and induced melanogenesis (M++++)) [30]. Melano-

cytes in a culture clearly differ from skin melanocytes; thus, this

system is a well-known, good experimental model for testing the

cellular response to environmental challenges [35].

UVB exposure reduced the viability by 40% in B16-F10 CT

(left), while pigmented cells (M++++, right) showed only a ,9%

reduction (Fig. 3). Therefore, the higher melanin content protect-

ed the pigmented cells from UVB damage, which is consistent with

previous data from the literature [30,36,37]. The effect of visible

light was opposite to the effect observed for UVB irradiation.

Furthermore, upon irradiation with visible light (36 J.cm22), the

control cells for both the Caucasian melanocytes (H36) and the

B16-F10 (M36) cells (left side, Fig. 3B) only showed a ,5%

decrease in cell viability. At 72 J.cm22, the viability decrease was

also small (i.e., 15%; M72, left side, Fig. 3B). However, when the

cells were pigmented and treated with 36 or 72 J.cm22 of visible

light, both cell lines exhibited substantial decreases in viability

(50% for H36, 25% for M36 and 40% for M72), which clearly

demonstrates that the presence of melanin increases visible light

phototoxicity. Moreover, the comet assay showed that the level of

DNA fragmentation was higher in the pigmented cells than in the

CT cells (Fig. 3B-images), which is consistent with increased visible

light phototoxicity for the increased level of intracellular melanin.

The mechanism of cell death was mainly necro-apoptosis;

substantial levels of propidium iodide were incorporated (cyto-

plasmic membrane damage, Fig. 3C), and caspase 3 was activated

(Fig. 3D).

To correlate the visible light phototoxicity with the melanin and1O2 contents, we quantified the amount of 1O2 generated in cells by

measuring the near-infrared emission spectrum (1270 nm) after

excitation with visible light (532 nm) [18] (Fig. 4A). The control cells

only exhibited background signals (Fig. 4A, insert). However, the

pigmented B16-F10 cells (M++++) showed the characteristic 1O2

spectra with a maximum intensity at approximately 1270 nm (the

darker line in the Fig. 4A insert). Thus, the higher intracellular

melanin content in the pigmented cells corresponded to more 1O2

generation and higher phototoxicity in response to visible irradiation

compared with those for the control cells. To establish a definitive

relationship between the 1O2 level and the cell toxicity, we also

generated this species in the intracellular environment using a clean1O2 source (i.e., thermal decomposition of DHPNO2). As shown in

Fig. 4B, the cells treated with DHPNO2 exhibited a substantial

decrease inviability (,50%)comparedwith theviabilityofboth types

of control (cells without treatment and cells challenged with DHPN),

which demonstrates that 1O2 plays a role in the reduced cell viability

(Fig. 4B). Furthermore, the generation of 1O2 slightly reduced the

viability of CT cells (49.7%) compared with that of M++++ cells

(53.3%). This difference was not statistically significant, possibly

because melanin can suppress 1O2. As reported earlier, melanin can

exertsboth typesof effects, but in the presenceof sufficient visible light

illumination,melaninwill stimulate thegenerationof 1O2 [14,18,38].

The excitation of melanin using visible light generates 1O2 and,

consequently, the triplet species derived from melanin. Therefore,

cellular damage can occur by both a type I mechanism (direct

reaction between the triplet photosensitizer and biological targets,

typically through an electron transfer reaction) and a type II

mechanism (energy transfer reaction between the triplet photo-

sensitizer and oxygen-forming 1O2) [19]. Depending on the

severity of the damage, cell death will be the main outcome from

visible light exposure. Another potential outcome, which is

potentially more dangerous, is the generation of oxidative DNA

products, which could lead to mutagenic compound accumulation,

genomic instability and cancer [39].

To demonstrate the direct damage to nuclear DNA by the melanin

photosensitization that occurs in response to visible light irradiation,

we performed a comet assay under low-dose conditions (i.e., a light

dose that does not measurably decrease the cell viability for both CT

and M++++ cells (6 J.cm22)). After irradiation, the cells were treated

withendonucleaseenzymes (FpgandEndoIII) thatrecognizespecific

types of oxidative damage in DNA (Fig. 5). Fpg recognizes 8-

oxoguanine, 8-hydroxyguanine and formamidopyrimidine, and

Endo III recognizes strand breaks, abasic sites and additional

oxidative pyrimidine modifications [40]. For a control, the comet

assay was repeated in the absence of endonucleases. As expected,

under this mild condition, both the CT and pigmented cells in the

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Figure 2. The effects of UVB and visible light on cell viability. The cell viability (MTT) of HaCaT, SK-mel and B16-F10 cells maintained in thedark (control) or irradiated with UVB (4.5 J.cm22) or visible (36 J.cm22) light. The color of the bars indicates the (qualitative) levels of melanin naturallyproduced by each cell line. Other cell lines, which are not melanocompetent (Hela, J774), were also tested and behaved similarly to the HaCaT cells(i.e., exhibited no phototoxicity from visible light at this dose (36 J.cm22)).doi:10.1371/journal.pone.0113266.g002

Figure 3. The effects of UVB and visible light in B16-F10 and human melanocyte cells. Left side: (no extra pigmentation); right side: cellssubjected to the pigmentation protocol (M++++). (A) Viable cells (%) determined using the acridine orange fluorescence of B16-F10 cells that weremaintained in the dark or with 4.5 J.cm22 of UVB irradiation. The images are confocal optical microscopy images of the B16-F10 cells, CT (left) andM++++ (right). (B) The viable cells (%) were determined using the MTT colorimetric assay in the dark or with 36 J.cm22 or 72 J.cm22 of visibleirradiation; murine B16-F10 cells are marked as either M36 or M72 depending on the light dose received. The human melanocytes (H36) only receiveda light dose of 36 J.cm22. The images on the sides show comet assays performed using the CT and M++++ B16-F10 cells, 180 min after irradiationwith a light dose of 36 J.cm22. (C) Propidium iodide incorporation in B16-F10 CT and M++++ cells in the dark and after treatment with 72 J.cm22 ofvisible light. The images on the sides show typical images used for quantifying the PI incorporation. (D) Caspase 3 activation in B16-F10 CT andM++++ cells in the dark and after exposure to 72 J.cm22 of visible light. The images on the side are typical for quantifying caspase 3 activation. (*) p,0.001.doi:10.1371/journal.pone.0113266.g003

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presenceandabsenceofvisible lightdidnotshowdirect strandbreaks,

as indicated by the comet assay (data not shown, except for the

M++++ cells with visible light irradiation, Fig. 5D). The first three

controls (i.e., CT in the presence and absence of light and M++++ in

the absence of light) also showed no DNA fragmentation after

treatment with Fpg and Endo III (Fig. 5A–C). As mentioned above,

the M++++ cells treated with light in the absence of these enzymes did

notshowDNAfragmentation (Fig. 5D).However,whenFpgorEndo

III were applied to M++++ cells, which were irradiated with 6 J.cm22

of visible light, the comet assay showed a considerable increase in the

number of strand breaks (Fig. 5 graph and images E–F), which were

absent in the controls (Fig. 5A–D). The presence of strand breaks

demonstrated that melanin photosensitization by visible irradiation

induces directoxidative damage to nuclearDNA.Theratio ofFpg- to

Endo III-sensitive modifications indicate that oxidative damage in

DNA is most likely due to both type II and type I mechanisms [32,33]

(Fig. 5, scheme).

Discussion

Although melanin is biosynthesized to protect against UVB, it

will damage skin cells in the presence of UVA and visible light. At

sufficiently high light doses, melanin causes extensive necro-

apoptotic cell death. The lower light doses were able to mimic the

potential chronic consequences of visible light exposure. Melanin

induces the formation of pre-mutagenic DNA lesions (Fpg- and

Endo III-sensitive modifications) (Fig. 4, scheme). We also showed

that pheomelanin is a more efficient photosensitizer than

eumelanin because it generates more 1O2 and better withstands

photobleaching (i.e., it continuously generates 1O2 for longer

periods than does eumelanin).

Therefore, in the presence of melanin, the effects of visible light

irradiation do not differ from those of UVA irradiation;

consequently, it should be considered with care, and further

investigations must be performed to evaluate whether it may be a

class I carcinogen [41]. These data indicate a causal relationship

between visible light irradiation and the development of genome

instability in melanocompetent cells and, consequently, the

development of melanoma [17,18,36] and the higher skin cancer

prevalence in individuals with red hair [15,37]. Other authors

have also concluded that visible light causes effects similar to those

of UVA, such as inflammation and ROS production [6,10,17].

The current beliefs regarding the protection of skin against

photoinduced damage are similar to those from 30 years ago but

with a shift in the problematic wavelength region. In the early

1980 s, photobiologists knew that UVA induced cellular responses

[2,3]. However, people were convinced sunbathing with UVB-

only protection was safe. The consequences of this strategy are felt

today: the resulting deeper skin tumors have a higher prevalence of

DNA mutations than those induced by UVA exposure [5,42].

Clearly, visible light affects skin health, but people are encouraged

to stay under the sun if they use sufficient amounts of ‘‘good

sunscreen’’ (i.e., sunscreens that provide effective protection

against UVA and UVB). This recommendation is clearly a

mistake because it ignores the effects of visible light, which

penetrates more deeply into skin than does UVB and UVA [43],

and because it disregards the effects of other wavelength regions,

such as the infrared region [44].

The toxicity of visible light raises concerns about other

situations in addition to the direct sun phototoxicity to the skin,

e.g., clinical protocols that use visible light, such as blue light

therapy, in jaundiced babies [45]; indoor tanning [46]; the

Figure 4. The effects of singlet oxygen on cells. (A) Phototoxicity and integrated emission spectra of the 1O2 that was generated uponexcitation at 532 nm as a function of intracellular melanin production (in mg melanin/mg of total protein, squares) in B16-F10 CT or M++++ cells.Insert: 1O2 emission spectra from B16-F10 (CT and M++++) cells. The excitation wavelength was 532 nm 5-ns laser pulses at 10 pulses/s and 1 mJ/pulse. (B) Chemical generation of 1O2 by thermal degradation of DHPNO2 in CT and M++++ cells. The data are represented as the means6SD of threeindependent experiments, and the results are expressed as the percentage of viable cells compared with that in the control group. (*p,0.05 and***p,0.001).doi:10.1371/journal.pone.0113266.g004

Melanin Photosensitization and Visible Light on Skin Cells

PLOS ONE | www.plosone.org 6 November 2014 | Volume 9 | Issue 11 | e113266

exposure of eyes to the toxicity of high levels of visible

environmental light [47].

In the presence of melanin, visible light generates singlet oxygen

and causes direct DNA damage. We hope this information will

guide health professionals and the general population to safer

interactions with the sun and, specifically, with visible light. We

also hope that this information encourages companies to develop

new sunscreen products that also provide protection against visible

radiation.

Supporting Information

Figure S1 Scheme of the melanin photosensitizationmechanisms that generate 1O2. This 1O2 can react with the

following to form several products: lipids mainly through an ene

reaction that forms hydroperoxide, nucleic acids via a guanine

residue to form 8-oxo-guanine, and amino acids (the scheme

shows the amino acids that are most reactive with 1O2). The right

side of the scheme shows the thermal decomposition of DHPNO2,

which is also used to generate 1O2 in the intracellular

environment.

(TIF)

Acknowledgments

Prof. S. Ito from Fujita Health University, Japan is acknowledged for

providing samples of eumelanin and pheomelanin.

Author Contributions

Conceived and designed the experiments: MSB. Performed the experi-

ments: OCN EA FFF. Analyzed the data: PDM MGM GRM SSME.

Contributed reagents/materials/analysis tools: MHM GRM MSB.

Contributed to the writing of the manuscript: MSB OCN. Read and

provided suggestions and corrections: ASF WKM CP DS FFF SSME

GRM EA PDM MHM.

Figure 5. Melanin photosensitization with visible light damages nuclear DNA. (Left) Comet assay. (A) B16-F10 CT cells in the dark aftertreatment with 0.2 U of Fpg (treatment with Endo III yielded the same results); (B) B16-F10 M++++ cells in the dark after treatment with 0.2 U of Fpg(treatment with Endo III yielded the same results); (C) B16-F10 CT cells exposed to 6 J.cm22 of visible irradiation after treatment with 0.2 U of Fpg(treatment with Endo III yielded the same results); (D) B16-F10 M++++ cells exposed to 6 J.cm22 of visible irradiation without enzymes; (E, F) the sameas D treated with 0.2 U of Endo III (E) and Fpg (F). The graphic on the left shows the quantification of the DNA fragmentation in B16-F10 cells underconditions A through E. (Right) A schematic of melanin photosensitization via type I and type II mechanisms, leading to damage in several biologicaltargets, including the cell membrane and nuclear DNA. The DNA changes are indicated by using the main Fpg and Endo III recognition sites.doi:10.1371/journal.pone.0113266.g005

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References

1. Yamaguchi Y, Takahashi K, Zmudzka BZ, Kornhauser A, Miller SA, et al.

(2006) Human skin responses to UV radiation: pigment in the upper epidermis

protects against DNA damage in the lower epidermis and facilitates apoptosis.

FASEB journal: official publication of the Federation of American Societies for

Experimental Biology 20: 1486–1488. Available: http://www.ncbi.nlm.nih.gov/

pubmed/16793869. Accessed 2014 June 19.

2. Vile GF, Tyrrell RM (1995) UVA radiation-induced oxidative damage to lipids

and proteins in vitro and in human skin fibroblasts is dependent on iron and

singlet oxygen. Free radical biology & medicine 18: 721–730. Available: http://

www.ncbi.nlm.nih.gov/pubmed/7750796. Accessed 2014 March 15.

3. Berneburg M, Grether-Beck S, Kurten V, Ruzicka T, Briviba K, et al. (1999)

Singlet oxygen mediates the UVA-induced generation of the photoaging-

associated mitochondrial common deletion. The Journal of biological chemistry

274: 15345–15349. Available: http://www.ncbi.nlm.nih.gov/pubmed/

10336420. Accessed 2013 Dec 23.

4. Cadet J, Douki T (2011) Oxidatively generated damage to DNA by UVA

radiation in cells and human skin. The Journal of investigative dermatology 131:

1005–1007. Available: http://www.ncbi.nlm.nih.gov/pubmed/21494240. Ac-

cessed 2014 June 19.

5. Agar NS, Halliday GM, Barnetson RS, Ananthaswamy HN, Wheeler M, et al.

(2004) The basal layer in human squamous tumors harbors more UVA than

UVB fingerprint mutations: a role for UVA in human skin carcinogenesis.

Proceedings of the National Academy of Sciences of the United States of

America 101: 4954–4959. Available: http://www.pubmedcentral.nih.gov/

articlerender.fcgi?artid=387355&tool=pmcentrez&rendertype=abstract. Ac-

cessed 2014 March 15.

6. Grether-Beck S, Marini A, Jaenicke T, Krutmann J (2014) Photoprotection of

human skin beyond ultraviolet radiation. Photodermatology, photoimmunology

& photomedicine 30: 167–174. doi:10.1111/phpp.12111.

7. Mahmoud BH, Hexsel CL, Hamzavi IH, Lim HW (n.d.) Effects of visible light

on the skin. Photochemistry and photobiology 84: 450–462. Available: http://

www.ncbi.nlm.nih.gov/pubmed/18248499. Accessed 2014 Jan 19.

8. Kielbassa C, Roza L, Epe B (1997) Wavelength dependence of oxidative DNA

damage induced by UV and visible light. Carcinogenesis 18: 811–816.

Available: http://www.ncbi.nlm.nih.gov/pubmed/9111219. Accessed 2014

Jan 19.

9. Kvam E, Tyrrell RM (1997) Induction of oxidative DNA base damage in human

skin cells by UV and near visible radiation. Carcinogenesis 18: 2379–2384.

Available: http://www.ncbi.nlm.nih.gov/pubmed/9450485. Accessed 2014 Jan

19.

10. Liebel F, Kaur S, Ruvolo E, Kollias N, Southall MD (2012) Irradiation of skin

with visible light induces reactive oxygen species and matrix-degrading enzymes.

The Journal of investigative dermatology 132: 1901–1907. Available: http://

www.ncbi.nlm.nih.gov/pubmed/22318388. Accessed 2013 Nov 10.

11. Mahmoud BH, Ruvolo E, Hexsel CL, Liu Y, Owen MR, et al. (2010) Impact of

long-wavelength UVA and visible light on melanocompetent skin. The Journal

of investigative dermatology 130: 2092–2097. Available: http://www.ncbi.nlm.

nih.gov/pubmed/20410914. Accessed 2013 Nov 19.

12. Kolbe L (2012) How much sun protection is needed?: Are we on the way to full-

spectrum protection? The Journal of investigative dermatology 132: 1756–1757.

Available: http://www.ncbi.nlm.nih.gov/pubmed/22695285. Accessed 2014

Jan 19.

13. Tadokoro T, Yamaguchi Y, Batzer J, Coelho SG, Zmudzka BZ, et al. (2005)

Mechanisms of skin tanning in different racial/ethnic groups in response to

ultraviolet radiation. The Journal of investigative dermatology 124: 1326–1332.

Available: http://www.ncbi.nlm.nih.gov/pubmed/15955111. Accessed 2014

Jan 19.

14. Suzukawa AA, Vieira A, Winnischofer SMB, Scalfo AC, Di Mascio P, et al.

(2012) Novel properties of melanins include promotion of DNA strand breaks,

impairment of repair, and reduced ability to damage DNA after quenching of

singlet oxygen. Free radical biology & medicine 52: 1945–1953. Available:

http://www.ncbi.nlm.nih.gov/pubmed/22401857. Accessed 2014 Jan 19.

15. Tadokoro T, Kobayashi N, Zmudzka BZ, Ito S, Wakamatsu K, et al. (2003)

UV-induced DNA damage and melanin content in human skin differing in

racial/ethnic origin. FASEB J 17: 1177–1179. Available: http://www.ncbi.nlm.

nih.gov/pubmed/12692083.

16. Setlow RB, Grist E, Thompson K, Woodhead AD (1993) Wavelengths Effective

in Induction of Malignant-Melanoma. Proceedings of the National Academy of

Sciences of the United States of America 90: 6666–6670. Available: ,Go to

ISI.://A1993LM68100060.

17. Wood SR, Berwick M, Ley RD, Walter RB, Setlow RB, et al. (2006) UV

causation of melanoma in Xiphophorus is dominated by melanin photosensi-

tized oxidant production. Proceedings of the National Academy of Sciences of

the United States of America 103: 4111–4115. Available: http://www.

pubmedcentral.nih.gov/articlerender.fcgi?artid=1449655&tool=pmcentrez&

rendertype=abstract. Accessed 2014 March 15.

18. Chiarelli-Neto O, Pavani C, Ferreira AS, Uchoa AF, Severino D, et al. (2011)

Generation and suppression of singlet oxygen in hair by photosensitization of

melanin. Free radical biology & medicine 51: 1195–1202. Available: http://

www.ncbi.nlm.nih.gov/pubmed/21723388. Accessed 2013 Nov 18.

19. Ogilby PR (2010) Singlet oxygen: there is indeed something new under the sun.

Chemical Society reviews 39: 3181–3209. Available: http://www.ncbi.nlm.nih.gov/pubmed/20571680. Accessed 2014 April 4.

20. Pierlot C, Aubry JM, Briviba K, Sies H, Di Mascio P (2000) Naphthalene

endoperoxides as generators of singlet oxygen in biological media. Methods in

enzymology 319: 3–20. Available: http://www.ncbi.nlm.nih.gov/pubmed/10907494. Accessed 2014 June 19.

21. Haywood RM, Lee M, Andrady C (2008) Comparable photoreactivity of hair

melanosomes, eu- and pheomelanins at low concentrations: low melanin a riskfactor for UVA damage and melanoma? Photochemistry and photobiology 84:

572–581. Available: http://www.ncbi.nlm.nih.gov/pubmed/18399925. Ac-

cessed 2014 Jan 19.

22. Ito S, Wakamatsu K (2003) Quantitative analysis of eumelanin and pheomelaninin humans, mice, and other animals: a comparative review. Pigment Cell Res 16:

523–531. Available: http://www.ncbi.nlm.nih.gov/pubmed/12950732.

23. Uchoa AF, Knox PP, Turchielle R, Seifullina Nk, Baptista MS (2008) Singletoxygen generation in the reaction centers of Rhodobacter sphaeroides. Eur

Biophys J 37: 843–850. Available: http://www.ncbi.nlm.nih.gov/pubmed/

18286272.

24. Fidler IJ (1975) Biological behavior of malignant melanoma cells correlated totheir survival in vivo. Cancer research 35: 218–224. Available: http://www.

ncbi.nlm.nih.gov/pubmed/1109790. Accessed 2014 Oct 9.

25. Boukamp P, Petrussevska RT, Breitkreutz D, Hornung J, Markham A, et al.(1988) Normal keratinization in a spontaneously immortalized aneuploid human

keratinocyte cell line. The Journal of cell biology 106: 761–771. Available:

http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=2115116&tool=pmcentrez&rendertype=abstract. Accessed 2014 Sept 20.

26. Ralph P, Nakoinz I (1975) Phagocytosis and cytolysis by a macrophage tumour

and its cloned cell line. Nature 257: 393–394. Available: http://www.ncbi.nlm.nih.gov/pubmed/1101071. Accessed 2014 Oct 9.

27. Carey TE, Takahashi T, Resnick LA, Oettgen HF, Old LJ (1976) Cell surface

antigens of human malignant melanoma: mixed hemadsorption assays for

humoral immunity to cultured autologous melanoma cells. Proceedings of theNational Academy of Sciences of the United States of America 73: 3278–3282.

Available: http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=431008&tool=pmcentrez&rendertype=abstract. Accessed 2014 Oct 9.

28. Brohem CA, Massaro RR, Tiago M, Marinho CE, Jasiulionis MG, et al. (2012)

Proteasome inhibition and ROS generation by 4-nerolidylcatechol inducesmelanoma cell death. Pigment cell & melanoma research 25: 354–369.

Available: http://www.ncbi.nlm.nih.gov/pubmed/22372875. Accessed 2014

Oct 9.

29. Collier AC, Pritsos CA (2003) The mitochondrial uncoupler dicumarol disruptsthe MTT assay. Biochemical pharmacology 66: 281–287. Available: http://

www.ncbi.nlm.nih.gov/pubmed/12826270. Accessed 2014 Dec 23.

30. Heo S-J, Ko S-C, Cha S-H, Kang D-H, Park H-S, et al. (2009) Effect ofphlorotannins isolated from Ecklonia cava on melanogenesis and their protective

effect against photo-oxidative stress induced by UV-B radiation. Toxicology

in vitro: an international journal published in association with BIBRA 23: 1123–1130. Available: http://www.ncbi.nlm.nih.gov/pubmed/19490939. Accessed

2013 Nov 30.

31. Bradford MM (1976) A rapid and sensitive method for the quantitation ofmicrogram quantities of protein utilizing the principle of protein-dye binding.

Analytical biochemistry 72: 248–254. Available: http://www.ncbi.nlm.nih.gov/

pubmed/942051.

32. Pouget JP, Douki T, Richard MJ, Cadet J (2000) DNA damage induced in cellsby gamma and UVA radiation as measured by HPLC/GC-MS and HPLC-EC

and Comet assay. Chemical research in toxicology 13: 541–549. Available:http://www.ncbi.nlm.nih.gov/pubmed/10898585. Accessed 2014 June 19.

33. Sauvaigo S, Petec-Calin C, Caillat S, Odin F, Cadet J (2002) Comet assay

coupled to repair enzymes for the detection of oxidative damage to DNA

induced by low doses of gamma-radiation: use of YOYO-1, low-backgroundslides, and optimized electrophoresis conditions. Analytical biochemistry 303:

107–109. Available: http://www.ncbi.nlm.nih.gov/pubmed/11906159. Ac-cessed 2014 June 19.

34. Olive PL, Banath JP (2006) The comet assay: a method to measure DNA

damage in individual cells. Nature protocols 1: 23–29. Available: http://www.

ncbi.nlm.nih.gov/pubmed/17406208. Accessed 2014 Jan 10.

35. Hearing VJ (2000) The melanosome: the perfect model for cellular responses tothe environment. Pigment cell research/sponsored by the European Society for

Pigment Cell Research and the International Pigment Cell Society 13 Suppl 8:23–34. Available: http://www.ncbi.nlm.nih.gov/pubmed/11041354. Accessed

2014 June 19.

36. Lin JY, Fisher DE (2007) Melanocyte biology and skin pigmentation. Nature445: 843–850. Available: http://www.ncbi.nlm.nih.gov/pubmed/17314970.

Accessed 2014 Jan 18.

37. Rees JL (2003) Genetics of hair and skin color. Annual review of genetics 37: 67–

90. Available: http://www.ncbi.nlm.nih.gov/pubmed/14616056. Accessed2014 Jan 19.

38. Beltran-Garcıa MJ, Prado FM, Oliveira MS, Ortiz-Mendoza D, Scalfo AC,

et al. (2014) Singlet molecular oxygen generation by light-activated DHN-melanin of the fungal pathogen Mycosphaerella fijiensis in black Sigatoka disease

Melanin Photosensitization and Visible Light on Skin Cells

PLOS ONE | www.plosone.org 8 November 2014 | Volume 9 | Issue 11 | e113266

of bananas. PloS one 9: e91616. Available: http://www.pubmedcentral.nih.

gov/articlerender.fcgi?artid=3960117&tool=pmcentrez&rendertype=abstract.ccessed 2014 June 19.

39. Russo MT, Blasi MF, Chiera F, Fortini P, Degan P, et al. (2004) The oxidized

deoxynucleoside triphosphate pool is a significant contributor to geneticinstability in mismatch repair-deficient cells. Molecular and cellular biology

24: 465–474. Available: http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=303369&tool=pmcentrez&rendertype=abstract. Accessed 2014 Jan

19.

40. Hatahet Z, Kow YW, Purmal AA, Cunningham RP, Wallace SS (1994) Newsubstrates for old enzymes. 5-Hydroxy-29-deoxycytidine and 5-hydroxy-29-

deoxyuridine are substrates for Escherichia coli endonuclease III andformamidopyrimidine DNA N-glycosylase, while 5-hydroxy-29-deoxyuridine is

a substrate for uracil DNA N-glycos. The Journal of biological chemistry 269:18814–18820. Available: http://www.ncbi.nlm.nih.gov/pubmed/8034633. Ac-

cessed 2014 Jan 19.

41. Dahle J, Kvam E (2003) Induction of delayed mutations and chromosomalinstability in fibroblasts after UVA-, UVB-, and X-radiation. Cancer research

63: 1464–1469. Available: http://www.ncbi.nlm.nih.gov/pubmed/12670891.Accessed 2014 Jan 19.

42. Halliday GM, Agar NS, Barnetson RSC, Ananthaswamy HN, Jones AM (2005)

UV-A fingerprint mutations in human skin cancer. Photochemistry and

photobiology 81: 3–8. Available: http://www.ncbi.nlm.nih.gov/pubmed/

15335275. Accessed 2014 March 15.

43. Mahmoud BH, Hexsel CL, Hamzavi IH, Lim HW (2007) Effects of visible light

on the skin. Photochemistry and photobiology 84: 450–462. Available: http://

www.ncbi.nlm.nih.gov/pubmed/18248499. Accessed 2014 March 6.

44. Schieke SM, Schroeder P, Krutmann J (2003) Cutaneous effects of infrared

radiation: from clinical observations to molecular response mechanisms.

Photodermatology, photoimmunology & photomedicine 19: 228–234. Available:

http://www.ncbi.nlm.nih.gov/pubmed/14535893. Accessed 2014 March 16.

45. Oplander C, Hidding S, Werners FB, Born M, Pallua N, et al. (2011) Effects of

blue light irradiation on human dermal fibroblasts. Journal of photochemistry

and photobiology B, Biology 103: 118–125. Available: http://www.ncbi.nlm.

nih.gov/pubmed/21421326. Accessed 2014 Jan 19.

46. Youssef PN, Sheibani N, Albert DM (2011) Retinal light toxicity. Eye (London,

England) 25: 1–14. Available: http://www.pubmedcentral.nih.gov/

articlerender.fcgi?artid=3144654&tool=pmcentrez&rendertype=abstract. Ac-

cessed 2014 Jan 19.

47. Fisher DE, James WD (2010) Indoor tanning–science, behavior, and policy. The

New England journal of medicine 363: 901–903. Available: http://www.

pubmedcentral.nih.gov/articlerender.fcgi?artid=3951814&tool=pmcentrez&

rendertype=abstract. Accessed 2014 March 16.

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